Scoliosis is a medical condition where the spine has a complex three-dimensional (3D) deformity. It is generally characterized by a lateral deviation of the spine, accompanied by an axial rotation of the vertebrae, deformation of the rib cage, and possibly of the pelvis. When observed on a two-dimensional (2D) posteroanterior (PA) or anteroposterior (AP) radiographic image (images obtained while the patient stands in the upright position and the x-rays directly radiate to the backside or the front side of the patient), the spine is curved like a C or an S. Scoliosis may be the result of an underlying congenital or developmental osseous or neurologic abnormality, but in most cases the cause is unknown (idiopathic scoliosis). The high-risk group for scoliosis are adolescents and symptoms typically show during their grow spurts at the age of 13-14. In total 2% to 3% of the population are affected by scoliosis[1]. Progressive scoliosis, if left untreated, can result in significant deformity that can further cause dangerous complications such as heart and lung problems (i.e. shortness of breath).
Depending on the severity of the deformation, treatment of scoliosis can involve regular observation, bracing and/or surgery. The most common treatment is that the patients wear a brace that applies corrective forces through the soft tissue of the trunk, rib cage and pelvis. Surgical treatment usually involves correction of the scoliotic curves with pre-shaped metal rods anchored in the vertebrae with screws or hooks, and arthrodesis (bone fusion) of the intervertebral articulations of the instrumented segment of the spine [2].
For diagnosis, monitoring, therapeutic planning, and epidemiologic analysis of scoliosis, images of the spine are required. Imaging modalities such as radiography, computed tomography (CT) and magnetic resonance (MR) imaging are commonly used, where radiography plays the primary role.
In a 2D radiographic image, the degree of scoliosis is determined by estimating the curvature of the spine. The curvature is commonly quantified by the Cobb angle. When assessing the curve condition, the apical vertebra, which is the furthest deviated vertebra (FIG. 1 A), is first identified. The Cobb angle (FIG. 1 B) is defined as the angle between a line parallel to the upper end-plate of the most tilted vertebrae above the apical vertebrae and a line parallel to the lower end-plate of the most tilted vertebra below the apical vertebra (see FIG. 1). In the illustrative example of FIG. 1, two lines are drawn wherein one line parallel to the upper end-plate of the most tilted vertebrae above the apical vertebrae (A) while the other line parallel to the lower end-plate of the most tilted vertebra below the apical vertebra. The Cobb angle (B) is measured as illustrated.
In S-shaped scoliosis, where there are two contiguous curves, the lower end vertebra of the upper curve will represent the upper end vertebra of the lower curve. The angle can be measured manually or digitally, and the methods have been found to be equally reliable[3]. The Scoliosis condition is defined as a lateral spinal curvature with a Cobb angle larger than 10°.
Using a measurement of the Cobb angle in a single 2D image as the only measurement of the degree of scoliosis has its limitations since it only reflects the curvature of the spine in a single plane. As a result, the actual Cobb angle might be up to 20% greater than that estimated from the radiographs[4]. A total error of 2°-7° can also be expected in the Cobb angle assessment due to the variations in radiographic acquisitions and measurement error[5]. Moreover, intra-observer variation by 5°-10° in the Cobb angle measurement has been reported, and the inter-observer variation is even greater[6,7]. Despite these drawbacks, measurement of the Cobb angle is still the gold standard when assessing the curve severity and the risk of curve progression in scoliosis patients.
CT or MR imaging is performed when the radiography alone is inadequate to identify an underlying cause of scoliosis. Although MR imaging is radiation-free, its performance is limited for cases with metal implants. Clinically, the use of CT is mandatory in cases of a complex osseous deformity. In addition to radiography, CT images of scoliosis provide information about the extent of rotation of the spine, segmentation defects, and detection of bony spur in diastomatomyelia and associated congenital anomalies of ribs, scapula and pelvis. Cross-sectional CT images are useful for guiding surgical treatment and evaluating post-operative complications, and 3D CT can provide more detailed images of the anterior and posterior components of the malformed vertebrae. Also, CT images can be used to assess the traditional Cobb angle [8].
Meanwhile, the radiation dose during the x-ray imaging of patients with scoliosis is a major concern, especially for CT scanning, since adolescents, who are the high-risk group of scoliosis, also have higher sensitivity to x-ray radiation. Many “radiation free” methods and systems for estimating the degree of scoliosis have therefore been proposed. For instance, in U.S. Pat. App. No. 20140303522, an evaluation system a using 3D position sensor system to capture the uneven state of the body surface is disclosed. The downside of the proposed system is that it cannot perform the estimation of the Cobb angle or the inner condition of the deformed spine. In U.S. Pat. App. No. 20110021914, a 3D ultrasound imaging system for assessing scoliosis is disclosed. The solution is composed of an ultrasound scanner, a spatial sensor and a software module. The usability of the solution is however limited by its operational complexity, the limited dimension of ultrasound scanner and the poor image quality of bone in ultrasonic images. Many protocols for minimization and optimization of the radiation dose during imaging of scoliosis have been proposed. For instance, the company “EOS® Imaging” provides an imaging system that acquires low-dose whole-body radiographic images at two vertical directions at the same time. The detector/source pairs are positioned with a fixed relative angle between each other. 3D images of the spine can be reconstructed from these two unique low-dose images, with no additional radiation. Due to the sensitivity of patient movement, a unique scanner is required for acquiring the two images.
There is thus a general need for improvements for enabling assessment of scoliosis.